Content to be shown on a display is often provided in encrypted form to reduce the risk of piracy (unauthorized duplication). For example, the High-bandwidth Digital Content Protection System (HDCP) is a commonly-used standard for encrypting video data by exclusive-ORing (XORing) the data with a pseudo-random bit stream (PRBS)—see Digital Content Protection, LLC, High-bandwidth digital content protection system, Revision 1.4, Jul. 8, 2009. Although HDCP protects data in transit between devices (e.g., between a DVD player and a TV using HDMI, the High-Definition Multimedia Interface), at some point within a display the data must be decrypted and fed to the pixels. Attackers (e.g., pirates) who can gain access to the data after the point of decryption can make a full pirate copy. Moreover, the master key for HDCP has recently been publically disclosed, so HDCP-encrypted content can now be freely decrypted by an attacker—see Mills, Elinor, “Intel: Leaked HDCP copy protection code is legit.” Sep. 16, 2010.
Flat-panel displays are commonly employed to display content transported in encrypted form. Liquid-crystal displays (LCD), plasma displays (PDP) and electroluminescent (EL) displays are examples of flat-panel displays. EL displays can be made from various emitter technologies, including coatable-inorganic light-emitting diode, quantum-dot, and organic light-emitting diode (OLED). EL emitters use current passing through thin films of EL material to produce light. EL displays employ both active-matrix and passive-matrix control schemes and can employ a plurality of pixels. Each pixel can include an EL emitter; drive transistors for driving current through the EL emitter are also provided on the display. The pixels are typically arranged in two-dimensional arrays with a row and a column address for each pixel, and having a data value associated with the pixel. Pixels can be of different colors, such as red, green, blue and white.
Initially, display systems used separate decryption ICs located off the display substrate. However, the outputs of the decryption ICs on such systems can readily be probed to pirate the content.
FIG. 9 shows a more recent method of protection against piracy employed in a conventional EL display. Display substrate 400 supports EL emitter 50 under cover 408. Cover 408 can be glass, metal foil or other materials known in the art. Seal 409 is used to prevent moisture from entering sealed area 16, the space between the substrate and the cover, as EL emitter 50 is damaged by moisture. Seal 409 can include an adhesive and desiccant as known in the art. Sealed area 16 includes display area 15, in which all of the EL emitters 50 are located. EL emitter 50 receives current through metal layer 403 from driver IC 420 via solder ball 421, or a wire bond (not shown) in pad-up configurations as known in the art. Driver IC 420 can be a chip-on-glass (CoG), flip-chip or BGA (Ball Grid Array) package, or a CSP (chip-scale package). Driver IC 420 is outside of the display area 15, and outside of sealed area 16. Glob-top 422, which can be an epoxy or other molding compound, covers driver IC 420 to make it difficult to remove driver IC 420 from display substrate 400. Driver IC 420 receives encrypted data and provides decrypted data to EL emitter 50. Glob-top 422 can be extended over metal layer 403 up to seal 409 to provide additional security. U.S. Patent Application Publication No. 2006/0158737 by Hu et al., paragraph 44, is an example of a related scheme. U.S. Pat. No. 6,442,448 by Finley et al., cols. 18-19, has further discussion of using epoxy to protect security-sensitive components. U.S. Patent Application Publication No. 2005/0201726 by Malcolm et al., paragraphs 54 and 81, also describes alternative ways of protecting transmissions of data inside a device.
However, glob-top is frequently used in the electronics industry for non-security applications, e.g., environmental resistance, and so rework tools have been developed which are capable of removing epoxy with minimal damage to the device(s) in question (here, display substrate 400 and driver IC 420). For example, section 5.4.3 of An Engineer's Handbook of Encapsulation and Underfill Technology by Martin Bartholomew (Electrochemical Publications Ltd 1999, ISBN 090115038X), entitled “Methods for encapsulant removal (decapsulation),” describes such methods. Automated Production Equipment of Key Largo, Fla., USA sells rework equipment that can be used to remove and replace glob-topped, conformally-coated, or underfilled BGA parts; see the 2009 article “Reworking Plastic Parts” from APE. Wayne Chen, in “FCOB reliability evaluation simulating multiple rework/reflow processes” (IEEE Transactions on Components, Packaging, and Manufacturing Technology, Part C, vol. 19, no. 4, pp. 270-276, October 1996), pg. 272, left column, describes epoxy encapsulant used as an underfill between a flip-chip IC and the PCB on which it is mounted, then simulates rework of that underfill to determine the reliability effects due to rework. For a conventional display as shown in FIG. 9, after glob-top 421 is reworked or removed, metal layer 403 can be probed to pirate the decrypted data.
Furthermore, display signals are generally high-frequency. An RGB 1920×1080p60 display has a 373 MHz pixel rate (1920 columns×1080 rows×3 (RGB)×60 Hz=373 MHz). The pixel signals therefore produce large amounts of high-frequency noise. This noise can be inductively probed through glob-top, permitting piracy without any need to destroy or rework the display.
Additionally, some image data is transported compressed instead of, or in addition to, encrypted. Existing systems for image decompression are also vulnerable to attack.
There is a need, therefore, for a more secure approach for decrypting or decompressing image data.